Expectations and placebo responses to caffeine-associated stimuli
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To test the theory that expectations control placebo responses.
Subjects (n=20) were asked how much they expected their arousal to increase after one or two cups of coffee, and were subsequently exposed to one or two cups of decaffeinated coffee, or to caffeine equivalent to one or two cups of coffee (200 and 400 mg). The expectancy theory of placebo responses predicts a positive correlation between expectations and actual placebo responses.
Dependent variables were acoustic startle eyeblink and skin conductance responses, blood pressure and heart rate, and measures of subjective arousal.
Caffeine increased startle eyeblink and skin conductance responses, as well as blood pressure and subjective arousal. Decaffeinated coffee increased startle eyeblink and skin conductance responses, but had no effect on subjective arousal, although the participants clearly expected increased subjective arousal after both one and two cups of coffee. However, there were significant correlations between the alertness expected after coffee, and the actual alertness recorded after decaffeinated coffee.
The main finding in this study was that relatively strong expectations about the effects of coffee did not generate placebo responses after administration of decaffeinated coffee. Expectations were dose dependent, whereas the placebo response was not. However, expected alertness after coffee predicted recorded alertness after coffee. In sum, the expectancy theory of placebo effects received only limited support.
KeywordsPlacebo response Caffeine Expectation Classical conditioning Startle Skin conductance
This research was funded by a grant from the Norwegian Science Council (project no. 142540/320) to Dr. Flaten.
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